The present invention relates to a process for preparing vinylphosphonic acid compounds using Pd(0)-catalysts, and to the use of such catalysts for the preparation process.
Vinylphosphonic acid compounds, in particular dialkyl vinylphosphonates, have importance as precursors for preparing vinylphosphonic acid and as monomers for copolymerization for producing adhesives or flame-resistant plastics.
Various processes for preparing them are known. In the process described in DE-C21 32 962, ethylene oxide is reacted with phosphorus trichloride to give 2-chloroethanephosphonic dichloride, and this compound is converted into bis-2-chloroethyl 2-chloroethanephosphonate. The resulting compound is then reacted with phosgene in the presence of a catalyst. Amines, heterocyclic nitrogen compounds, as well as tertiary phosphines, are used as catalyst.
DE-A-30 01 894 describes a process for preparing vinylphosphonic acid derivatives, in which dialkyl 2-acetoxyethanephosphonates are cleaved in the presence of acidic or basic catalysts. The basic catalysts proposed are tertiary amines and phosphines, as well as ammonium salts or phosphonium salts, besides heterocyclic compounds and amides. The disadvantage of the process is the formation of a mixture of vinylphosphonic acid derivatives. The maximum content of dialkyl vinylphosphonates is 23%.
An improved variant of this process discolosed in DE-A-31 20 437 entails a distillation followed by reaction of the bottom product mixture resulting from the distillation with orthocarboxylates to give dialkyl vinylphosphonates.
EP-A-0 722 948 discloses thermal cleavage of diethyl 2-acetoxyethanephosphonate in the gas phase to give acetic acid and dimethyl vinylphosphonate. No catalyst is used in this case.
The disadvantages of the above processes are the formation of product mixtures, elaborate, multistage synthetic processes, the need to use high reaction temperatures, and the use of chlorinated starting compounds. The large proportion in particular considerably impairs the economics of the process.
A simple addition reaction is advantageous for synthesizing vinylphosphonic acid compounds and results in the required product in high yields. One example of a reaction of this type is addition of dialkyl phosphites onto acetylene. U.S. Pat. No. 3,673,285 describes the addition of alkynes onto diethyl phosphite at from 130 to 200xc2x0 C. in the presence of nickelphosphine complexes. On addition of acetylene, the corresponding diethyl vinylphosphonate is obtained in a yield of 30%. The disadvantage of this process is, besides the low yield, the tendency of the phosphorous esters to decompose in a strongly exothermic reaction at temperatures as low as 130xc2x0 C.
It is an object of the present invention to provide a process for preparing vinylphosphonic acid compounds which avoids the disadvantages of known processes and makes the required products available with high selectivity and yield under mild conditions from acetylene and phosphorous acid compounds.
We have found that his object is achieved by a process for preparing vinylphosphonic acid compounds of the formula (I) 
where R1 and R2 are, independently, H, C1-16-alkyl, C6-12-aryl, C7-12-alkaryl or C7-12-aralkyl, it being possible for the organic radicals to be substituted by one or more halogen atoms, hydroxyl, acyl or acetoxy groups, by reacting phosphorous acid compounds of the formula (II) 
where R1 and R2 have the above meaning, with acetylene in the presence of a Pd(0) complex or a Pt(0) complex as catalyst.
We have found that the reaction of acetylene wit phosphorous acid compounds, in particular dialkyl phosphites, to give vinylphosphonic acid compounds, in particular dialkyl vinylphosphonates, is possible with high selectivity and yield by using a catalytic amount of a Pd(0) or Pt(0) complex, preferably Pd(0) complex, especially in homogeneous phase. Palladium complexes have been used for hydrophosphorylation of terminal higher alkynes such as 1-octyne, see J. Am. Chem. Soc., 118 (1996) 1571-1572. However, reaction with acetylene is not mentioned or proposed here.
The hydrophosphorylation of higher alkynes and also of acetylene is disclosed in U.S. Pat. No. 5,693,826. According to this reference, various Pd(II) and Pd(0) complexes can be used as catalyst. In the examples of this reference, there is made use of Pd(II) complexes exclusively, with 2 exceptions. These show that Pd(0) complexes give lower yields and require longer reaction times, respectively, compared to Pd(II) complexes. When acetylene is used as a starting product, the yields are very low (20%).
We have now found that acetylene can be reacted under very mild conditions with very high selectivity using a Pd(0) catalyst to give vinylphosphonic acid compounds, in particular dialkyl vinylphosphonates, directly without any dimerization, oligomerization or polymerization of acetylene or double reaction to give a tetraalkyl ethylenediphosphonate.
It is supposed that the low yield mentioned above is due to one or several of these side reactions occurring while the hydrophosphorylation reaction is carried out. Acetylene can be said to have a different reactivity, compared to substituted acetylenes like, for example, 1-octyne. When Pd(0) complexes are used in the hydrophosphorylation reaction, is as surprisingly found that high conversion and high selectivities could be obtained, contrary to the use of Pd(II) complexes.
In the phosphorous acid compounds of the formula (II) employed for the reaction, R1 and R2 are, independently, H, C1-16-alkyl, C6-12-aryl, C7-12-alkaryl or C7-12-aralkyl, it being possible for the organic radicals to be substituted by one or more halogen atoms, hydroxyl, acyl or acetoxy groups. R1 and R2 are preferably, independently, linear C1-12-alkyl, phenyl, (C1-6-alkyl)phenyl or phenyl(C1-6-alkyl).
R1 and R2 are particularly preferable, independently, linear C1-6-alkyl radicals. These radicals are preferably unsubstituted.
If R1 and R2 differ from hydrogen, the compounds employed are diesters of phosphorous acid. Reaction thereof results in diesters of vinylphosphonic acid of formula (I). This reaction can be followed by cleavage of the ester groups, resulting in vinylphosphonic acid, R1OH and R2OH.
Conversion of the phosphorous acid compounds into the vinylphosphonic acid compounds takes place in the presence of a Pd(0) complex or Pt(0) complex, but preferably Pd complex as catalyst. The catalyst is usually present in homogeneous phase for this purpose.
The Pd(0) complex employed preferably has phosphine ligands or phosphite ligands. A large number of ligands are suitable as phosphine ligands or phosphite ligands. For example, the ligands may have the formula PXYZ where X, Y and Z are, independently, alkyl, aryl, alkoxy or aryloxy radicals having up to 18 carbon atoms. Alkyl or aryl radicals are preferred in this connection, especially aryl radicals. Corresponding ligands are described, for example, in DE-A.1 593 277. They are preferably triarylphosphines or triaryl phosphites in which the aryl groups are unsubstituted or substituted. Suitable substituents are C1-6-alkyl, acyl or acetoxy radicals. The triarylphosphine or triarylphosphite is preferably unsubsituted. Triphenylphosphine is particularly employed as phosphine ligand. The catalyst used particularly preferably according to the invention is tetrakis(triphenylphosphine)palladium(0).
The complexes may be composed, for example, of monodentate or bidentate ligands. Examples of a suitable complex structure is the following: 
where the meanings are
M Pd, Pt, preferably Pd
R independently at each position organic radicals linked via O and/or C atoms to the phosphorus atoms, in particular aryl radicals or aryloxy radicals having 2 sites capable of linkage.
R1 independently at each position monovalent organic radicals, in particular aryl and/or aryloxy radicals.
It is preferred for the monovalent radicals to be derived from benzene or phenol and for the divalent radicals to be derived from biphenyl, 1,1xe2x80x2-binaphthyl, biphenyloxy and/or 1,1xe2x80x2-binaphthyloxy radicals. It is moreover possible for all the aromatic radicals to be substituted, for example by one or more C1-6-alkyl radicals or corresponding alkoxy radicals. The biphenyl and binaphthyl radicals, and radicals derived therefrom, are linked to the phosphorus atom via 2 positions in the molecule. It is possible for both positions to be linked to the same phosphorus atom. It is also possible for them to be linked to different phosphorus atoms and thus produce bridged structures which have, for example, 2 phosphorus atoms and 3 of said radicals. Suitable corresponding bidentate phosphite ligands are described in U.S. Pat. No. 5,512,695. The phosphite ligands described therein can also be employed in analogous form as phosphine ligands. Further suitable monodentate and bidentate aromatic ligands are described in WO 95/29153. The described ligands can in this case likewise be employed as phosphine or phosphite ligands.
The catalysts can moreover be formed in situ in the reaction.
The catalysts employed according to the invention are normally employed in an amount of from 0.01 to 10% by weight, preferably in an amount of from 0.5 to 3% by weight, particularly preferably 1 to 2% by weight, based on the amount of phosphorous acid compounds to be vinylated, in particular dialkyl phosphites.
The temperature in the reaction is, as a rule, from 20 to 120xc2x0 C., preferable 20 to 80xc2x0 C., particularly preferably 60 to 80xc2x0 C.
The reaction can moreover be carried out without solvent or in the presence of an inert solvent. Examplex of inert solvents which can be used are cyclic ethers such as THF, long-chain ethers such as triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether.
The reaction is carried out under atmospheric pressure or elevated pressure, preferably at from 1 to 20, particularly preferably 1.5 to 6, bar (absolute). This preferably entails mixing the phosphorous acid compound of the formula (II) and the catalyst, and passing in acetylene. Once the reaction is complete, the product can be removed by distillation.
The novel process can be carried out continuously or batchwise.
The invention also relates to the use of the catalysts described above in the preparation of vinylphosphonic acid compounds, in particular vinylphosphonic esters, specifically dialkyl vinylphosphonates.
The invention is explained in details by means of Examples shown hereafter.